For the purpose of promoting the acid-neutralizing effect of metallic detergent-dispersant additives(metal type detergents)ents in lubricating oils, succinimide compounds are combined with the additives and added to various lubricating oils. As one example, hereinunder referred to is the use of succinimide compounds in lubricating oil for diesel engines.
In general, since fuel to be used in diesel engines has a large sulfur content, diesel engine exhaust gas contain a large amount of SOx (sulfur oxides). The SOx reacts with water vapor in the exhaust gas to give sulfuric acid. The sulfuric acid thus formed in large quantities breaks the oil film of the lubricating oil being used in diesel engines, thereby corroding and wearing the parts constituting the engines. In particular, in 2-cycle engines for ships, the cylinder liners and the piston rings are much corroded and worn by the acid. Therefore, it is extremely important for diesel engine lubricating oil to have the ability of resisting such corrosion and wear. Heretofore, a suitable amount of a perbasic, metallic detergent-dispersant additives has been added to lubricating oil, by which sulfuric acid formed in the oil is neutralized, resulting in that cylinder liners and piston rings are protected from being corroded or worn. However, the recent diesel engines in land vehicles are equipped with an exhaust gas recirculation (EGR) system for the purpose of reducing the amount of exhaust gas to be discharged from the engines, resulting in that the SOx content of the exhaust gas from those EGR-mounted engines tends to increase over that from the conventional diesel engines. Even in diesel engines for ships, low-quality fuel having a high sulfur content has become much used in these days for reducing the fuel cost. Therefore, the SOx content of the exhaust gas from those engines also tends to increase, like that from the recent diesel engines in land vehicles. Such a large amount of sulfuric acid as formed through the reaction of much SOx and water vapor could not be neutralized to such a degree that the thus-neutralized acid does no more corrode and wear engine parts, even if the amount of the perbasic, metallic detergent-dispersant additives to be added to engine lubricating oil is merely increased. Given that situation, a combination of the perbasic, metallic detergent-dispersant additives and an ashless dispersant has been tried, thereby promoting the acid-neutralizing effect of the metallic detergent-dispersant additives in order to protect the cylinder liners and piston rings of engines from being corroded and worn. For example, Japanese Patent Application Laid-Open No. 6-271885 discloses an additive comprising calcium salicylate and a bis-type alkenylsuccinimide in its Example, in which, however, the alkenylsuccinimide used requires the addition of an amine-molybdenum complex thereto in order to attain the intended object. Therefore, the development of succinimide compounds of a novel type is desired.
On the other hand, it is known that, in diesel engines, the acid-neutralizing speed of the metallic detergent-dispersant additives used has some relation to the corrosion and wear of the cylinder liners and the piston rings constituting the engines. Therefore, the acid-neutralizing speed of the agent in lubricating oil used in diesel engines shall indicate the ability of the lubricating oil of protecting cylinder liners and piston rings from being corroded and worn.